Draft gears, widely used in railroad industry to provide protection to a railroad car by absorbing shocks in both draft and buff conditions, must meet various Association of American Railroads (AAR) requirements. The draft gear must be capable of maintaining the minimum shock absorbing capacity during its service life required by AAR standard M-901-E to be at least 36,000 foot pounds. Also the AAR mandates that working action of such draft gear is to be achieved without exceeding a 500,000 pound reaction pressure acting on the freight car sills in order to prevent upsetting the coupler shank. Further, the draft gear must pass a drop hammer test meeting the endurance portion of the AAR standard M-901-G, which determines the shock absorbing capacity of the draft gear.
The commonly used draft gears, installed horizontally in alignment with a railroad car center, include a housing having a front and a rear portion. A compressible cushioning element is positioned within the rear portion of the housing. A friction cushioning element is in the front portion of the housing adjacent the coupler of such railroad car. A spring release mechanism is provided for continuously urging the friction cushioning element outwardly from the compressible cushioning element to release such friction cushioning element after compression of such draft gears. The compressible cushioning element is typically either an all spring configuration or a spring and hydraulic assembly combination as taught in U.S. Pat. No. 3,368,698.
The draft gear employing a hydraulic assembly, enables a higher drop hammer capacity than an all spring design and is capable of a higher shock absorbing capacity.
patent application Ser. No. 10/634,559 teaches a draft gear with a shock absorbing capacity to be slightly higher than 100,000 foot pounds, capable of achieving a higher protection to the railroad car prior to the draft gear using all of its travel. A feature of this draft gear is related to the fixed size of the metering pin area and, more particularly, a fixed opening pressure of the hydraulic metering system which is not affected by the high fluid pressure side in combination with the spring force required to keep the valve in the closed position.
However, it was discovered, such draft gear did not meet the requirements of the drop hammer test due to such fixed size of the metering pin area. An attempt to modify the size of the metering pin area to meet drop hammer test requirements resulted in an acceptable performance on the test track simulating operational buff conditions. As it was further determined, the underlying cause is related to a difference in a speed of impact between a drop hammer test and operational buff condition application. Such speed differential caused variations in the hydraulic pressure resulting in performance variations between the drop hammer test and operational buff conditions.
U.S. Pat. Nos. 5,529,194; 5,152,409; and 4,645,187 all owned by the assignee of the present invention, teach various improvements of the friction cushioning element disposed in the front portion of the draft gear housing suitable for use with the hydraulic compressible cushioning element disposed in the rear portion of the housing.
U.S. Pat. No. 6,488,162 to Carlstedt teaches another embodiment of the friction cushioning element suitable for use with the hydraulic compressible cushioning element.
Additionally, U.S. Pat. No. 6,446,820 to Barker et al teaches a compressible resilient member, comprising an elastomer element, installed in a front portion of the draft gear adjacent the coupler shank and suitable for use with the hydraulic compressible cushioning element.